More and more cities worldwide are working on their digital transformation in order to deliver a better living environment for citizens. Over the last decades, a lot of research efforts are dedicated to the development of semantic 3D city models, covering a variety of city dimensions based on standards such as CityGML. The complexity of cities’ processes, the heterogeneity of data they produced and different special scale requirements for planning and managing urban environments bring emerging challenges to the implementation of semantic 3D city models. In order to address these challenges, this paper presents the process of developing a semantic 3D model of the city of Sofia and its visual representation through a simple web application. As a pilot area, District “Vazrajdane” is selected, consisting of several neighbourhoods close to the centre of the city. The 3D transformation of proprietary geospatial data into CityGML schema, covering building and terrain modules, is described. Further, the implementation of the web application for user interaction with the semantic 3D city model is presented. Its main features include silhouetting a single building on handling mouse events and showing relevant overlay content, displaying shadows and styling of buildings depending on their data attributes.